This invention relates to a transistor protection circuit comprising:
an output transistor of a first conductivity type comprising a collector, base and emitter,
a driver transistor of the first conductivity type comprising a collector, base and emitter, the emitter being coupled to the base of the output transistor,
a current-sensing transistor of the first conductivity type comprising a collector, base and emitter, having its base and emitter coupled to the base and the emitter, respectively, of the output transistor, its collector current being a measure of the collector current of the output transistor, and
means coupled to the current-sensing transistor for limiting the drive current to the output transistor depending on the collector current of the current-sensing transistor.
Such a circuit may be used, for example, in audio and video circuitry and in motor-control circuits.
Such a circuit is known from inter alia, U.S. Pat. No. 4,330,757. In this circuit the current through the output transistor is measured by means of a current-sensing transistor whose base-emitter junction is arranged in parallel with that of the output transistor, the collector current of the current-sensing transistor being a fraction of the collector current of the output transistor. By means of a current mirror the collector current of the current-sensing transistor is reflected to a resistance element which is arranged in parallel with the base-emitter junction of a clamping transistor, the collector of the clamping transistor being connected to the base of the driver transistor. In the case of an excessive collector current of the current-sensing transistor the clamping transistor is driven into conduction so that, what would otherwise be a part of the base current of the driver transistor is drained via the clamping transistor.
A disadvantage of this known circuit arrangement is that in integrated form it exhibits a high degree of instability in the protection loop. These instabilities are caused by phase shifts in the current mirror arranged in the protection loop, which current mirror comprises PNP-transistors which have a comparatively low cut-off frequency.